Negative stiffness hydraulic system

ABSTRACT

A variable stiffness structure configured to support a variable load, the variable stiffness structure including a positive stiffness element coupled to the variable load, a negative stiffness element, a hydraulic system coupled to the positive and negative stiffness elements and configured to adjust a relative position of the positive and negative stiffness elements in response to a change in the variable load, while the variable stiffness structure supports the variable load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication No. 61/913,163, filed Dec. 6, 2013, and is related to a U.S.patent application entitled “Negative Stiffness System with VariablePreload Adjustment,” U.S. application Ser. No. 14/335,800, filed on Jul.18, 2014, the entire contents of both applications are incorporated byreference herein.

FIELD

The following description relates generally to the stiffness ofnon-linear structures and, more particularly, to a negative stiffnesshydraulic systems.

BACKGROUND

Negative stiffness can be generated by non-linear behavior. Forinstance, simple and widely used non-linear structures that can generatenegative stiffness include snap-through beams, buckling beams,over-rotation, and rolling or sliding contact between components.Non-linear structures that exhibit both positive and negative stiffnessare potentially useful in a variety of mechanical design applications.For instance, a negative stiffness element (e.g., a buckling-type beamthat can exhibit non-linear behavior) can be combined with a positivestiffness element, such as a supporting spring, to provide a structurehaving zero or quasi-zero stiffness (QZS) over a range of displacements.The quasi-zero stiffness of the structure may be used (utilized) toisolate another object or mass (e.g. a structure, device, package,and/or an instrument) from unwanted vibrations because the transmissionof vibrations through systems of very low stiffness is minimal. However,these related art isolating structures tend to be unstable in theirisolating mode and are not easily utilized for their damping and shockisolation abilities, because manufacturing technologies and techniquesare typically not accurate enough to create a QZS structure that doesnot require frequent tuning. Additionally, conventional QZS structuresmay not be capable of maintaining low to zero stiffness in the presenceof large static load changes (e.g., large static force changes), whichmay induce a large displacement in the system. One approach is to use amotor to adjust a secondary, softer positive spring in parallel to themain support spring. However, this approach has the limitation of onlybeing able to adjust to small changes in force. Another approach uses(utilizes) a passive negative stiffness system where a large motionwould cause the negative stiffness element to disengage and slip inrelation to the positive element. However, this system has adisadvantage that it is unable to reset to its minimum stiffness afterslippage, which limits its ability to isolate unwanted vibrations.

Accordingly, what is desired is a low-cost, robust solution forre-centering negative stiffness without completely disengaging thenegative stiffness element.

SUMMARY

Aspects of embodiments of the present invention are directed to a systemcapable of isolating vibrations while still supporting large staticforces.

Aspects of embodiments of the present invention are directed towardvarious hydraulic isolation systems capable of isolating low-amplitudevibrations while concurrently undergoing large variations in staticforce. According to an embodiment, the hydraulic isolation systemcombines a controllable negative stiffness element with a hydraulicsystem, which provides a continuously variable adjustment of theconnectivity between the negative stiffness element and some externalpositive stiffness system, to account for variations in static forcethrough the negative stiffness element. According to another embodiment,compression in a passive negative stiffness element is maintained whilethe system is adjusted in response to variations in static force.

According to some embodiments of the present invention, there isprovided a variable stiffness structure configured to support a variableload, the variable stiffness structure including: a positive stiffnesselement coupled to the variable load; a negative stiffness element; anda hydraulic system coupled to the positive and negative stiffnesselements and configured to adjust a relative position of the positiveand negative stiffness elements in response to a change in the variableload, while the variable stiffness structure supports the variable load.

In an embodiment, the positive stiffness element is configured to couplethe variable load to an external body, and the negative stiffnesselement is configured to isolate vibrations of the variable load fromthe external body.

In an embodiment, the change in the variable load produces adisplacement in the relative position of the variable load and theexternal body exceeding an operational range of displacement withinwhich the negative stiffness element provides a negative stiffnessconstant to an aggregate stiffness constant of the variable stiffnessstructure, and, in response to the change in the variable load, thehydraulic system is configured to return the variable stiffnessstructure to the operational range of displacement by applying fluidicpressure to adjust the relative position of the positive and negativestiffness elements.

In an embodiment, the variable stiffness structure is configured tomaintain a substantially constant stiffness as the relative position ofthe positive and negative stiffness elements is adjusted in response toa change in the variable load.

In an embodiment, the substantially constant stiffness is about zerostiffness.

In an embodiment, the positive stiffness element is configured toprovide a positive stiffness constant to an aggregate stiffness constantof the variable stiffness structure.

In an embodiment, the hydraulic system includes an actuator and isconfigured to disengage the negative stiffness element prior toadjusting the relative position of the positive and negative stiffnesselements, and to re-engage the negative stiffness element after theadjusting the relative position of the positive and negative stiffnesselements.

In an embodiment, the hydraulic system includes: a hydraulic chambercoupled to the negative stiffness element; an accumulator; and a valvesystem configured to move fluid between the hydraulic chamber and theaccumulator, wherein the hydraulic chamber is configured to exertfluidic pressure on the negative stiffness element to adjust theposition of the negative stiffness element relative to the positivestiffness element, in response to the change in the variable load.

In an embodiment, the valve system is configured to increase or decreasethe fluidic pressure inside the hydraulic chamber according to thechange in the variable load.

In an embodiment, the valve system includes two or more pressure reliefvalves arranged in opposite directions and configured to permit flow offluid between the hydraulic chamber and the accumulator, when a fluidpressure in either the hydraulic chamber or the accumulator exceeds arelief pressure.

In an embodiment, the two or more pressure relief valves include passivevalves.

In an embodiment, the valve system includes one or more of a passivevalve and an active valve.

In an embodiment, the hydraulic system includes: a first hydraulicchamber coupled between the negative stiffness element and the variableload; a second hydraulic chamber coupled between the negative stiffnesselement and a body; and a valve system configured to move fluid betweenthe first hydraulic chamber and the second hydraulic chamber to adjustthe position of the negative stiffness element relative to the positivestiffness element, in response to the change in the variable load.

In an embodiment, the valve system is configured to adjust the positionof the negative stiffness element relative to the positive stiffnesselement by reducing a fluidic pressure bias between the first and secondhydraulic chambers.

In an embodiment, the valve system includes one or more of a passivevalve and an active valve.

In an embodiment, the variable stiffness structure further includes asensor coupled to one of the first and second hydraulic chambers,wherein the sensor is configured to sense a fluid pressure inside one ofthe first and second hydraulic chambers, and wherein the valve system isfurther configured to control the stiffness of the negative stiffnesselement according to the sensed fluid pressure.

In an embodiment, the positive stiffness element includes a first rubberdisc and a second rubber disc, and is coupled to the variable loadthrough an inner post.

In an embodiment, the negative stiffness element includes a pair ofbuckled discs having a first end coupled to a body and a second endconfigured to slide along a length of the inner post.

In an embodiment, the stiffness of the NS element is controlled by anactuator coupled to the first end or second end of the negativestiffness element.

In an embodiment, the hydraulic system includes: a first hydraulicchamber between the first rubber disc and the negative stiffnesselement; a second hydraulic chamber between the first rubber disc andthe negative stiffness element; and a valve system configured to movefluid between the first hydraulic chamber and the second hydraulicchamber to adjust the position of the negative stiffness elementrelative to the inner post, in response to the change in the variableload.

In an embodiment, a stiffness of the negative stiffness element remainssubstantially constant as the valve system adjusts the position of thenegative stiffness element relative to the inner post.

In an embodiment, the hydraulic system includes: a first hydraulicchamber coupled between the negative stiffness element and the variableload; a second hydraulic chamber coupled between the negative stiffnesselement and a body; a first accumulator; and a valve system configuredto move fluid between the first accumulator and the first and secondhydraulic chambers, wherein the each of the first and second hydraulicchambers is configured to exert fluidic pressure on the negativestiffness element to adjust the position of the negative stiffnesselement relative to the positive stiffness element, in response to thechange in the variable load.

In an embodiment, the valve system is configured to increase or decreasefluidic pressures inside the first and second hydraulic chambersaccording to the change in the variable load.

In an embodiment, the valve system is configured to balance fluidicpressures between the first and second hydraulic chambers according tothe change in the variable load.

In an embodiment, the variable stiffness structure further includes asensor coupled to one of the first and second hydraulic chambers,wherein the sensor is configured to sense a fluid pressure inside one ofthe first and second hydraulic chambers, and wherein the valve system isfurther configured to control the stiffness of the negative stiffnesselement according to the sensed fluid pressure.

In an embodiment, the hydraulic system further includes a first actuatorconfigured to push fluid from one of the first and second hydraulicchambers into another one of the first and second hydraulic chambers.

In an embodiment, the hydraulic system further includes a secondactuator configured to increase or decrease a fluidic pressure in thefirst accumulator.

In an embodiment, the hydraulic system further includes: a secondaccumulator, wherein the valve system is further configured to movefluid between the second accumulator and the first and second hydraulicchambers.

According to some embodiments of the present invention, there isprovided an isolation system including: a body; a variable load; and avariable stiffness structure coupled to the body and the variable loadand configured to isolate vibrations of the variable load from the bodyin presence of a change in the variable load, the variable stiffnessstructure including: a positive stiffness element coupled to thevariable load; a negative stiffness element; and a hydraulic systemcoupled to the positive and negative stiffness elements and configuredto adjust a relative position of the positive and negative stiffnesselements in response to a change in the variable load, while thevariable stiffness structure supports the variable load.

In an embodiment, the hydraulic system includes: a hydraulic chambercoupled to the negative stiffness element; an accumulator; and a valvesystem configured to move fluid between the hydraulic chamber and theaccumulator, wherein the hydraulic chamber is configured to exertfluidic pressure on the negative stiffness element to adjust theposition of the negative stiffness element relative to the positivestiffness element, in response to the change in the variable load.

In an embodiment, the hydraulic system includes: a first hydraulicchamber coupled between the negative stiffness element and the variableload; a second hydraulic chamber coupled between the negative stiffnesselement and the body; a valve system configured to move fluid betweenthe first hydraulic chamber and the second hydraulic chamber to adjustthe position of the negative stiffness element relative to the positivestiffness element, in response to the change in the variable load.

Accordingly, embodiments of the present invention are capable ofmaintaining low stiffness (i.e., high isolation) at mid to highvibrational frequencies while permitting large loads to pass through theisolation system.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used in limiting the scope of theclaimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of embodiments of the presentinvention will become more apparent by reference to the followingdetailed description when considered in conjunction with the followingdrawings. In the drawings, like reference numerals are used throughoutthe figures to reference like features and components. The figures arenot necessarily drawn to scale.

FIG. 1A illustrates a force-displacement relationship 100 a for anegative stiffness element.

FIG. 1B illustrates the force-displacement relationship 100 b for ageneric quasi-zero-stiffness (QZS) spring system including a negativestiffness element and positive stiffness element.

FIG. 2 is a functional representation of a hydraulic isolation system,according to an illustrative embodiment of the present invention.

FIGS. 3A and 3B illustrate the concept of re-adjusting the connectivityof a negative stiffness element in an adaptive quasi-zero-stiffness(QZS) system to maintain the low-stiffness region at a current staticforce, according to illustrative embodiments of the present invention.

FIG. 4A is a functional representation of a hydraulic isolation systemutilizing a single-sided, passive, low-pressure hydraulic system,according to an illustrative embodiment of the present invention.

FIG. 4B is a functional representation of a hydraulic isolation systemutilizing a single-sided, active, low-pressure hydraulic system,according to an illustrative embodiment of the present invention.

FIG. 4C is a functional representation of a hydraulic isolation systemutilizing a single-sided, hybrid low-pressure hydraulic system havingboth active and passive valves, according to an illustrative embodimentof the present invention.

FIG. 5 is a schematic diagram of a hydraulic isolation system utilizinga dual-sided, hybrid, high-pressure hydraulic system, according to anillustrative embodiment of the present invention.

FIG. 6 is a schematic diagram of a mount in a hydraulic isolation systemutilizing a dual-sided, hydraulic system, according to an illustrativeembodiment of the present invention.

FIG. 7 is a schematic diagram of a dual-sided hydraulic isolation systemfor adjusting connectivity of a negative stiffness element withoutaltering the negative stiffness value, according to an illustrativeembodiment of the present invention.

DETAILED DESCRIPTION

In many mechanical structures, such as structures utilized intransportation systems, it is desirable to isolate broadband vibrationswhile being subjected to a wide range of static forces (e.g., g-loads).The present disclosure is directed to various embodiments of a largelypassive system (e.g., a variable stiffness structure) capable ofisolating vibrations while still supporting large static forces.

The variable stiffness structures of the present disclosure may beincorporated into any system or device in which it is desirable toprevent or at least reduce the transmission of vibration through thestructure or device, such as, for instance, gyroscopes (wherein theaccuracy of the gyroscope is proportional to the level of vibrationisolation), passenger vehicles (e.g., vibration isolation between theengine and the chassis of the vehicle or between a wheel and the road),aircraft (e.g., vibration isolation between a helicopter blade and a hubof the helicopter), cameras (wherein vibration reduction may improveimage quality), radar and other sensitive measurement equipment,precision targeting equipment (e.g., smart munitions targeting systems),sensitive medical equipment, satellites, and/or the like.Transmissibility is a measure of vibration isolation quality and isdefined as the ratio of the response amplitude of the isolated system ordevice to the excitation amplitude input into the system or device(i.e., the excitation amplitude is the force or displacement that thevariable stiffness structures of the present disclosure are configuredto prevent or at least reduce from being transmitted to avibration-sensitive structure).

In conventional dynamic systems it is often difficult to account forlarge variations in static force (e.g., “DC offset” in load). “DCoffset” in load refers to any load change where the frequency componentof the change is less than ⅕ of the natural frequency of the overallsystem. Embodiments of the present invention are directed toward ahydraulic isolator with quasi-zero-stiffness (QZS) stiffness capable ofisolating low-amplitude vibrations while allowing large variations instatic force.

In some embodiments, low stiffness (e.g., high isolation) is maintainedat mid to high frequencies while permitting DC loads to pass through thenetwork by utilizing a nearly passive network to reduce (e.g., minimize)the power used by the system. According to an embodiment, the hydraulicisolation system combines a controllable negative stiffness element witha hydraulic system, which provides a continuously variable adjustment ofthe connectivity between the negative stiffness element and someexternal positive stiffness system, to account for variations in staticforce through the negative stiffness element. According to anotherembodiment, compression in a passive negative stiffness element ismaintained while the system is adjusted in response to variations instatic force. Accordingly, embodiments of the present invention achievewide bandwidth isolation with high static (e.g., “DC”) stiffness andreduced (e.g., minimal) applied power.

FIG. 1A illustrates a force-displacement relationship 100 a for anegative stiffness element. FIG. 1B illustrates the force-displacementrelationship 100 b for a generic quasi-zero-stiffness (QZS) springsystem including a negative stiffness element and positive stiffnesselement.

Negative stiffness structures are structures that are in some unstablestate, for example, in a state of buckling or collapse. Some examplesare a buckled beam or an over-center toggle mechanism. Outside acritical displacement, the stiffness of the structure is positive, butwithin the critical range, stiffness is negative (represented by K_(n)in FIG. 1A). The negative stiffness may take any shape, but generallylies between a cubic (e.g., the dotted-line curve 102) and linear (e.g.,the solid-line curve 104) relationship with respect to displacement, asshown in FIG. 1A. When a negative stiffness structure is coupled inparallel with a positive stiffness structure (e.g., a spring having alinear force-displacement relationship represented by curve 112), thesystem may stabilize into a nonlinear quasi-zero-stiffness (QZS) system,exhibiting low stiffness within a limited force range 114, and highstiffness outside of that range, as illustrated by curve 110 in FIG. 1B.Such a passive system is useful in applications where the average forcestays within the “low stiffness” range, and the relationship betweenvibration amplitude and frequency is fixed. Examples may includemachines or equipment operating on the ground or a fixed surface.

For systems with a changing vibration spectrum or additionalenvironmental loads and accelerations, an adaptive negative stiffnesscomponent may be controlled (e.g., adjusted) to assume any stiffnessbetween the positive stiffness spring (e.g., curve 112) and nearly zerostiffness (e.g., near center portion of curve 110). While this may beuseful in situations where the static force is small and of limitedduration, when the low frequency or quasi-static force is large and moreprolonged the load may operate away from the low stiffness portion ofthe force displacement curve 110, thus passing more vibration throughthe network.

FIG. 2 is functional representation of a hydraulic isolation system(e.g., a hydraulic QZS system or hydraulically coupled negativestiffness system) 200, according to an illustrative embodiment of thepresent invention. The isolation system 200 includes a mass (e.g., avariable load) 201, a structure (e.g., a body) 212, and a mount 205 forprotecting (e.g., isolating) the mass 201 from the motion of thestructure 212 (as may be the case in, e.g., a camera mount), forprotecting (e.g., isolating) the structure 212 from a vibrating mass201, or, in some instances for protecting both (e.g. in engine mounts).The mount 205 may be represented, in part, as, a damper element 207, anda positive stiffness element (e.g., positive stiffness spring or staticstiffness element) 208. The damper element 207 and the positivestiffness (PS) element 208 represent the dissipation and load-supportingproperties, respectively, of the mount 205, and may, for example, be aviscous damper and a coil spring, or a solid piece of rubber. The damper207 and the positive stiffness element 208 shown in FIG. 2 are those ofa simple single-degree of freedom network, which may be translational ortorsional. However, embodiments of the present invention are not limitedthereto, and the spring and damper combination shown may be replacedwith a more complex positive-stiffness system.

In an embodiment, the mount 205 further includes a negative stiffnesselement (e.g., a variable negative stiffness) 206 and a hydraulic system(e.g., hydraulic clutching mechanism) 220. Without the hydraulic system220, the negative stiffness (NS) element 206 allows the isolation system200 to exhibit any stiffness between zero and that of the PS element208, provided that the static force exerted in the mount 205 remainssubstantially constant (e.g., within the low stiffness range 114 of FIG.1B). However, according to embodiments of the present invention, in theevent of large static force offsets (e.g., offsets that would cause theisolation system 200 to fall out of the low stiffness range 114 of FIG.1B), the hydraulic system 220 provides the isolation system 200 with alow-energy mechanism for adjusting the range (in force or displacement)of the NS element 206 to compensate for a static force offset. In FIG.2, the NS element 206 is schematically shown as a simple beam pinned ateach end. However, the NS element 206 is not limited thereto and mayinclude any complex negative element, such as a Belleville washer,buckling beam or column, higher-mode (2, 3) buckling beam, a bucklingdisc, a multi-link mechanism, and/or the like. The hydraulic system 220may be single-sided, i.e., coupled between only one end of the NSelement 206 and a corresponding one of the mass 201 and the structure212, or the hydraulic system 220 may be dual-sided, i.e., coupled toboth ends of the NS element 206 separating it on both ends from the mass201 and the structure 212 (e.g., as shown by the dotted end of thehydraulic system 220 in FIG. 2).

FIGS. 3A and 3B illustrate the concept of re-adjusting the connectivityof a negative stiffness element in an adaptive quasi-zero-stiffness(QZS) system to maintain the low-stiffness region at a current staticforce, according to illustrative embodiments of the present invention.FIG. 3A illustrates the force-displacement relationship of an adaptiveQZS system having a cubic negative stiffness element (such as shown bycurve 102 of FIG. 1A), according to an illustrative embodiment of thepresent invention. FIG. 3B illustrates the force-displacementrelationship of an adaptive QZS system having a linear negativestiffness element (such as shown by curve 104 of FIG. 1B), according toan illustrative embodiment of the present invention.

In FIGS. 3A and 3B, curves 300 a and 300 b represent the baselineforce-displacement relationship of the adaptive QZS system, according toembodiments of the present invention. The effective (or aggregate) lowstiffness is given by K_(d) that is the sum of the static stiffnessK_(s) (which stabilizes the system and supports the static load, F_(s))and negative stiffness K_(n) over the range of displacement where thenegative element provides a negative stiffness constant. When the staticforce F_(s) changes (e.g., due to additional mass or inertial loadseffects), the adaptive QZS system alters the relative position betweennegative and positive stiffness springs to adjust the region of lowstiffness to match a new value of F_(s). Curves 302, 304, 312, and 314represent examples of other force-displacement relationships that theadaptive QZS system may assume by changing the relative displacement ofthe negative and positive elements of the system. As shown in FIGS. 3Aand 3B, the adjustment creates a system with high effective staticstiffness for variable low frequency loads, and simultaneously orconcurrently low dynamic stiffness for higher frequency vibratory loads.

FIGS. 4A-4C are functional representations of single-sided hydraulicisolation systems (e.g., single-sided hydraulically coupled NS systems)400-1 to 400-3 for static and dynamic load filtering, according toillustrative embodiments of the present invention.

FIG. 4A is a functional representation of a hydraulic isolation system400-1 utilizing a single-sided, passive, low pressure hydraulic system420-1, according to an illustrative embodiment of the present invention.A low-pressure system may be one in which hydraulic fluid is allowed tohave a pressure below atmospheric pressure. In an embodiment, a mass 201is coupled to a structure (e.g., a body) 212 through a positivestiffness (PS) element (e.g., a positive spring) 208 and damper 207.While the damper 207 and PS element 208 represent a simple single-degreeof freedom network, such as a steel or rubber structure, embodiments ofthe present invention are not limited thereto and the damper/PS elementcombination may be replaced with any more complex positive stiffnesssystem. The negative stiffness (NS) element (e.g., a controllablenegative stiffness spring) 206 is coupled in parallel with the PSelement.

In an embodiment, the NS element 206 is coupled to (e.g., rigidlycoupled to) the structure 212 on one end as is coupled to a hydraulicchamber (e.g., a hydraulic column) 422 of a hydraulic system 420-1 onanother end. The mass 201 is coupled to (e.g., affixed to) the hydraulicchamber 422, which may have a fixed volume, constraining the mass 201 tobe coupled to the NS element 206. In an alternative embodiment, the NSelement 206 is coupled to (e.g., rigidly coupled to) the mass 201 andthe hydraulic chamber 422 is coupled between the NS element 206 and thestructure 212.

In an embodiment, the hydraulic chamber 422 is coupled to a low pressure(e.g., below atmospheric pressure) hydraulic accumulator (e.g., fluidreservoir) 424 through a passive valve system 426. In an example, thehydraulic accumulator 424 may be held at substantially zero pressure orat an other low pressure.

In an embodiment, the hydraulic fluid includes glycol, esters,organophosphate ester, polyalphaolefin, propylene glycol, silicone oils,and/or the like. For example, the hydraulic fluid includes oils,butanol, esters (e.g. phthalates, such as DEHP, and adipates, such asbis(2-ethylhexyl) adipate), polyalkylene glycols(PAG), organophosphate(e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons,polyalphaolefins (e.g. polyisobutenes), corrosion inhibitors (includingacid scavengers), anti-erosion additives, and/or the like.

According to an embodiment of the present invention, each time thestatic force exerted by the load or mass 201 changes beyond a threshold(e.g., a predetermined threshold), the isolation system 400-1 respondsto the change by increasing or decreasing the volume of fluid inside thehydraulic chamber 422. The isolation system 400-1 may detect the changein the static force in a number of ways, including determining whetherthe displacement of the NS element 206 exceeds a threshold (above orbelow its midpoint) for a length of time (e.g., a preset length oftime), determining whether pressure within the hydraulic system 420-1(e.g., within the hydraulic chamber 422) exceeds a threshold (e.g., apreset threshold) for a length of time, determining whether a forcedetected by a sensor (e.g., a transducer) between the NS element 206 andthe structure 212 exceeds a threshold (above or below zero) for a lengthof time, and/or the like.

The time component of the measurements may allow the isolation system400-1 to filter out sudden shocks. Depending on the application of thehydraulic isolation system 400-1, the length of time may vary from zeroseconds to several minutes. The time component of the measurements maybe implemented by, for example, a digital filter (e.g., when amicroprocessor controls the system), an analog filter across the sensorsignal (such as a capacitor across the output of a transducer), or aphysical filter (such as a restricted flow to a pressure transducer).

In an embodiment, following the detection of a change in the staticforce, the isolation system 400-1 may readjust (e.g., reset) itself tothe new force level by, for example, re-centering the NS element 206. Inso doing, the isolation system 400-1 may disengage (e.g., release) theNS element 206 (giving the NS element 206 zero or slightly positivestiffness), equalize pressure in the hydraulic system 420-1 (e.g.,equalize pressure between the hydraulic chamber 422 and the accumulator424) to, for example, atmospheric pressure, and then re-engage the NSelement 206. Thus, in an embodiment, the hydraulic isolation system400-1 repositions the NS element 206 both in terms of force andposition.

In an embodiment, the engaging/disengaging of the NS element 206 isperformed by one or more actuators coupled to (e.g., operatively coupledto) the NS element 206. The one or more actuators may also control thestiffness of the NS element, and may be located at any suitable position(e.g., at an end of the NS element) between the NS element 206 and themass 201 or the NS element 206 and the structure 212.

According to an embodiment of the present invention, two or morepressure relief valves (e.g., passive pressure relief valves) 426 arearranged in opposite directions and are configured to permit flow offluid (in opposite directions) between the hydraulic chamber 422 and theaccumulator 424 when the fluid pressure exceeds a relief pressure Pr.The value of the relief pressure Pr is dependent on the application ofthe hydraulic isolation system 400-1, which may be supporting anythingfrom a single instrument to an entire aircraft. In an embodiment, therelief pressure Pr is greater than the maximum force generated by thenegative stiffness element 206 (e.g., half of the peak-to-peak forcemagnitude shown FIG. 1A) divided by the hydraulic column area (e.g.,area of the hydraulic column 422). In an embodiment, the isolationsystem 400-1 senses when one or more of the pressure relief valves areopening and resets the NS element 206 by, for example, disengaging(e.g., releasing) the NS element 206 and returning the NS element 206 toits neutral position (e.g., its original centered position, when thevariable load or mass 201 is substantially constant) while the one ormore pressure relief valves 426 are still open. Once the NS element 206is reengaged and the one or more pressure relief valves close, thenegative stiffness of the isolation system 400-1 (e.g., the NS element206) is increased. In an example, this process may happen quickly and“quasi continuously” because the NS element 206 may be engaged anddisengaged rapidly. In an example, the rapid adjustment may occur atabout 2 Hz; however, this may occur as fast as 100 Hz. The reliefpressure (and hence the tolerated relief pressure delta) may be setaccording to the stable displacement range of the NS element 206 (e.g.,the displacement range corresponding to the low stiffness range 114 ofFIG. 1B) and the pressure differential range of the negative element,which may be at or above the maximum force generated by the negativestiffness element 206 (e.g., half of the peak-to-peak force magnitudeshown FIG. 1A) divided by the area of the hydraulic column 422.

FIG. 4B is a functional representation of a hydraulic isolation system400-2 utilizing a single-sided, active, low-pressure hydraulic system420-2, according to an illustrative embodiment of the present invention.

The isolation system 400-2 illustrated in FIG. 4B is substantiallysimilar to the isolation system 400-1 described above with respect toFIG. 4A, with the exception that the hydraulic isolation system 400-2utilizes an active hydraulic system, instead of a passive one.

In an embodiment, the hydraulic isolation system 400-2 performs the sameengage/release procedure described above, however, the timing of theopening/closing of the active one or more pressure relief valves 428 andthe engagement/disengagement of the NS element 206 is directlycontrolled by a control unit 430, which responds to a load/mass 201change as described above with respect to FIG. 4A. While the one or morepressure relief valves 428 are active, the fluid pressure of thehydraulic system 420-2 is still driven by the load/mass 201 and pumpsmay not be needed to maintain pressure in the accumulator 424. Theaccumulator 424 may be a high-pressure accumulator (e.g., a reservoirhaving fluid pressure higher than an atmosphere).

FIG. 4C is a functional representation of a hydraulic isolation system400-3 utilizing a single-sided, hybrid low pressure hydraulic system420-3 having both active and passive valves 426 and 428, according to anillustrative embodiment of the present invention.

The elements utilized in the isolation system 400-3 illustrated in FIG.4B are substantially similar to those of the isolation systems 400-1 and400-2 described above with respect to FIGS. 4A-4B, and a description ofthe elements and their operation may not be repeated.

According to an embodiment, the active valve 428 may handle lower staticload changes (which may entail low fluid velocities), while the passivevalves 426 may be used to adjust for higher static load changes. Forexample, the active valve 428 may be triggered at static loads at orbelow twice the maximum negative stiffness force (i.e., the maximumforce generated by the negative stiffness element 20 or the peak-to-peakforce magnitude shown FIG. 1A), while the passive valve 428 may betriggered at static loads greater than twice the maximum negativestiffness force.

In an embodiment, the relief pressure of the passive valve 426 is set tocorrespond to a force higher than the maximum negative stiffness forceto force, to ensure that the passive valve 426 only opens when thesystem is stable. If a valve opens while the hydraulic isolation system400-3 is in an unstable region (between the force peaks, as shown inFIG. 1A, or in the low stiffness region 114) then the negative stiffnesselement 206 may snap through and all isolating properties may be lostuntil the system 400-3 is reset. The active valve 428 may ensure thatthe system 400-2 operates in a stable region (e.g., at close to zerostiffness) and prevent the system 400-2 opening a valve in an unstableregion.

FIG. 5 is a schematic diagram of a hydraulic isolation system 500utilizing a dual-sided, hybrid, high-pressure hydraulic system 520,according to an illustrative embodiment of the present invention.

The dual-sided hydraulic isolation system 500 illustrated in FIG. 5 issimilar to the single-sided isolation systems 400-1 to 400-3 describedabove with respect to FIGS. 4A-4C, with the exception that the isolationsystem 500 utilizes a higher pressure system 520 to accommodate largerforces (e.g., larger load/mass 201 change) as compared to theembodiments of single-sided isolation systems 400-1 to 400-3.

According to an embodiment, instead of a hydraulic accumulator (e.g., afluid reservoir), the dual-sided isolation system 500 includes a secondhydraulic chamber (e.g., a hydraulic column) 523 at another end (e.g.,the lower side) of the NS element 206. In an embodiment, a positivepressure bias across the two hydraulic chambers 522 and 523 reduces(e.g., prevents) cavitation and allows the isolation system to adjust tohigher forces (as compared to a single-sided isolation system) from theNS element 206 that may be pulling on the fluid chamber. Rather thancouple the upper hydraulic chamber 522 to an accumulator, in anembodiment, the chamber 522 is coupled to the lower hydraulic chamber523, and the isolation system moves (e.g., shuttles) fluid between theupper and lower chambers 522 and 523 to maintain the compressive forceof the NS element 206 while allowing the supported mass/load 201 or thestructure (e.g., a body) 212 to displace against the PS element 208.

In an embodiment, the isolation system 500 adjusts (e.g., adapts) to achange in the static force (e.g., resulting from a change in theload/mass 201) by disengaging the NS element 206 from the mass 201,equalizing hydraulic pressure in the hydraulic system 520, andreengaging the NS element 206, in substantially the same manner as thatdescribed above with respect to FIGS. 4A and 4B.

While FIG. 5 depicts a hybrid hydraulic system including an activepressure relief valve 524 and a passive pressure relief valve 526, insome embodiments of the present invention, the hydraulic system may notinclude either the passive valve 526 or the active valve 528.

FIG. 6 is a schematic diagram of a mount 605 of an isolation systemutilizing a dual-sided, hydraulic system, according to an illustrativeembodiment of the present invention.

In an embodiment, the dual-sided isolation system includes a mount 605for supporting a load/mass 201 that is coupled to (e.g., affixed) themount 605 through a post (e.g. a rigid inner post or central shaft) 602.The mount 605 may be coupled to the structure (e.g., a body) through thesides (e.g., the left and right sides) of the mount housing 611 andserves to reduce (e.g., prevent) vibration transmission between themass/load and the structure. In an embodiment, the mount 605 includesupper and lower hydraulic chambers (e.g., first and second chambers orhydraulic columns) 622 and 623 and a DC offset valve 627, which mayinclude an active and/or passive pressure relief valve as describedabove with respect to FIGS. 4A-4C and 5. The DC offset valve 627 servesto equalize fluid pressure in the upper and lower chambers 622 and 623by allowing the transfer of fluid between the two chambers 622 and 623when the pressure differential of the two chambers 622 and 623 exceeds athreshold. In an embodiment, the threshold may be a pressurecorresponding to a force at or greater than the maximum Negativestiffness force and below five times the maximum negative stiffnessforce. In an embodiment, the pressure is each chamber is monitored by apressure sensor, which may be positioned in the chamber.

In an embodiment, a high-stiffness element (e.g., a high-stiffnessrubber mount) 608 acts as both the positive stiffness element and thedamper and may include a pair of rubber discs, and may be coupled, atthe center, to the post 602 and, at the outer boundary, to the mounthousing 611. The NS element 606 may include a pair of buckled discs andmay be coupled to (e.g., anchored to) the structure through the mounthousing 611 on one end (e.g., the base), while the other end of the NSelement 606 may float freely between the hydraulic chambers 622 and 623and not be attached to the rigid post 602. In an example, the floatingend of the NS element 606 is configured to slide along the length of thepost 602 via, for example, an O-ring or another fluid seal device. In anembodiment, each of the upper and lower hydraulic chambers 622 and 623includes a cavity formed between one side of the high-stiffness element608 and a corresponding side of the NS element 206. Thus, one side ofeach of the upper and lower hydraulic chambers 622 and 623 may becoupled to (e.g., rigidly coupled to) the load/mass 201 through thehigh-stiffness element 608.

The stiffness of the NS element 606 may be controlled by an actuator 610coupled (e.g., operatively coupled) between the base of the NS element606 and the structure.

FIG. 7 is a schematic diagram of a dual-sided hydraulic isolation system700 for adjusting connectivity of a negative stiffness element withoutaltering the stiffness of the NS element, according to an illustrativeembodiment of the present invention.

The dual-sided hydraulic isolation system 700 illustrated in FIG. 7 issimilar to the single-sided isolation systems 400-1 to 400-3 and thedual-sided isolation system 500, described above with respect to FIGS.4A-4C and FIG. 5, respectively, and includes similar elements,descriptions of which may not be repeated here.

According to some embodiments, the dual-sided hydraulic isolation system700 utilizes a hydraulic system 720 capable of responding to staticforce imbalance without altering the stiffness of the NS element 206, bysimply adjusting the position of the NS element 206 relative to themass/load 201 by the desired amount.

The hydraulic system 720 includes a first and second hydraulic chambers(first and second hydraulic columns) 722 and 723, each coupled to one ormore of a high-pressure accumulator (e.g., a high-pressure reservoir)726 a and a low-pressure high-pressure (e.g., a low-pressure reservoir)726 b through two or more of valves (e.g., controllable one-way valves)724 a-b and 725 a-b. Each of the valves 724 a-b and 725 a-b may becontrolled to operate in three states: closed (e.g., OFF), open (ON) andallowing flow in one direction, and open (ON) and allowing flow in theopposite direction. The hydraulic system 720 may further include a firstactuator 728, which may concurrently (e.g., simultaneously) push/pull onhydraulic chambers 722 and 723 (e.g., draw fluid from one the hydraulicchamber and push into the other), and a second actuator 730, which maybe utilized to raise or lower the pressure in the high-pressureaccumulator 726 a. In an embodiment, the actuator 728 may include two ormore antagonistic pistons for pushing in opposite directions, thusachieving a push/pull effect.

The hydraulic system 720 is inactive (“locked down”) whenever theisolation system 700 is properly adjusted (e.g., there is no staticimbalance) and the NS element 206 is oscillating about its neutralposition (e.g., centered position or fully compressed state).

In the event of a static force imbalance, the hydraulic system 720 maydetect the imbalance by utilizing one or more pressure sensors in thefirst and second hydraulic chambers 722 and 723, a displacement sensoron the NS element 206, and/or the like. In an embodiment, the NS element206 may be actively driven back to its centered position by one or moreof the first and second actuators 728 and 730. According to anembodiment (e.g., a “high-force” system), valve 724 a equalizes pressurebetween hydraulic chambers 722 and 723, and then closes. Next, the firstactuator 728 may push sufficient amount of fluid from the firsthydraulic chamber 722 to the second hydraulic chamber 723 (or viceversa) to return the NS element 206 to its neutral position. In anembodiment in which the NS element 206 rests against a “hard stop” whenout of balance, the throw of the first actuator 728 is fixed, and asolenoid may be used as the actuator. However, embodiments of thepresent invention are not limited thereto and any other actuator may beused (utilized), including pumps, motors, servo motors, shape memoryactuators, pneumatic actuators, or the like. According to an embodiment(e.g., a “low-force” system), the second actuator 730 may be used(utilized) in lieu of, or in addition to, the first actuator 728 tobring the NS element 206 to its neutral position (while valve 724 a isopen).

According to some embodiments of the present invention, the hydraulicsystem 720 adjusts the NS element 206 to a neutral position without anyexternal energy (to move hydraulic fluid) by storing pressure from theDC offset or system vibration in accumulator 726 a, and then using thestored pressure in a controlled manner to return the NS element 206 to aneutral position. Initially, the accumulator 726 a may be at a lowpressure. In an example, the accumulator 726 a may be equalized to thepressure in accumulator 726 b though valve 732. When the NS element 206is detected as out of position with a high pressure in one of hydraulicchambers 722 and 723, a valve 724 a or 725 a may open to transfer thispressure to accumulator 726 a. Once the pressure inside the accumulator726 a reaches a threshold pressure, valve 724 a or 725 b may close, andvalve 724 b or 725 b may equalize the pressure in the hydraulic chambers723 and 724 to a low pressure, and then close. Finally, the NS element206 may be pushed back to a neutral position by again opening valve 724a or 725 a and the opposite low-pressure valve 724 b or 725 b, using thepressure stored in accumulator 726 a to overcome the negative stiffnessof NS element 206. Valves 724 a-b and 725 a-b may be closed when theneutral position is reached.

In the event of only minor (or moderate) changes in static force, theremay not be enough built-up pressure in the hydraulic chamber 722/723 tosufficiently charge the accumulator 726 a. To add further robustness tothe hydraulic system 720, the second actuator 730 may be utilized toincrease the pressure inside the accumulator 726 a to compensate forinsufficient pressure buildup in the hydraulic chamber 722/723. Inaddition to, or in lieu of, the second actuator 730, the first actuator728 may be used (utilized) to perform substantially the same function asthe accumulator 726 a when there is insufficient pressure in thehydraulic chamber 722/723.

In an embodiment, one or more of the first and second actuators 728 and730 may be coupled to a large related art pressurized hydraulic orpneumatic system to provide faster and/or more precise response to anystatic force imbalance. In an example, a position sensor may detect theposition of the NS element 206, and a servo-valve (using systempressure) may drive the NS element 206 to its neutral position. Aclosed-loop control system, such as one described above, may besufficient to maintain isolation, and the actuators 728 and/or 730 mayonly move at the rate of the low-frequency load change (e.g., staticload change).

According to some embodiments of the present invention, the hydraulicfluid may be incompressible (i.e., its volume remains substantiallyconstant as pressure varies). However, embodiments of the presentinvention are not limited thereto and may, instead, include a suitablecompressible fluid.

In embodiments of the present invention, the hydraulic isolation systemmay include one or more actuators of any suitable kind, such as, activematerials (e.g., piezoelectric materials, shape memory alloys,magnetostrictive materials, electro-active polymers, and dielectricelastomers), servo motors, stepper motors, solenoids, ultrasonic drives,voice coils, hydraulics, wedges, levers, tapered shafts, and/or thelike.

According to some embodiments of the present invention, one or moresensors are configured to measure or detect one or more conditions ofthe hydraulic isolation system, such as, for instance, a position of theisolated mass/load, a position of the one or more stiffness elements(e.g., NS and/or PS elements), strain on the one or more stiffnesselements, fluid pressure inside the one or more hydraulic chambers, thetemperature of the hydraulic isolation system, and/or the like.Additionally, the one or more sensors may be configured to detect and/ormeasure one or more conditions of the structure or system into which thehydraulic isolation system is integrated. For instance, in an embodimentin which the hydraulic isolation system is incorporatd in a vehicle, theone or more sensors may be configured to collect external informationsuch as, engine revolutions per minute (RPMs), velocity, braking,steering inputs and/or the like. The one or more sensors may be any kindof sensors suitable for detecting and/or measuring the relevantconditions of the NS clutch system, such as, position sensors (e.g.,linear variable differential transformer (LVDT) sensors, opticalsensors, and/or laser-based sensors), strain sensors, load cells (e.g.,strain gauges in a Wheatstone bridge configuration) to provide loadinformation for the one or more stiffness elements (e.g., NS and/or PSelements), a temperature sensor to compensate for thermal effects,and/or the like. The one or more sensors may be configured to send oneor more signals to a control system, which drives the actuators toexpand or contract based upon the one or more input signals from the oneor more sensors.

While this invention has been described in detail with particularreferences to illustrative embodiments thereof, the embodimentsdescribed herein are not intended to be exhaustive or to limit the scopeof the invention to the exact forms disclosed. Persons skilled in theart and technology to which this invention pertains will appreciate thatalterations and changes in the described structures and methods ofassembly and operation can be practiced without meaningfully departingfrom the principles, spirit, and scope of this invention, as set forthin the following claims and equivalents thereof. Although relative termssuch as “outer,” “inner,” “upper,” “lower,” and similar terms have beenused herein to describe a spatial relationship of one element toanother, it is understood that these terms are intended to encompassdifferent orientations of the various elements and components of theinvention in addition to the orientation depicted in the figures.Additionally, as used herein, the term “substantially,” “about,” andsimilar terms are used as terms of approximation and not as terms ofdegree, and are intended to account for the inherent deviations inmeasured or calculated values that would be recognized by those ofordinary skill in the art. Furthermore, as used herein, when a componentis referred to as being “on” another component, it can be directly onthe other component or components may also be present therebetween.Moreover, when a component is component is referred to as being“coupled” to or “connected” to another component, it can be directlyattached to the other component or intervening components may be presenttherebetween.

What is claimed is:
 1. A variable stiffness structure configured tosupport a variable load, the variable stiffness structure comprising: apositive stiffness element coupled to the variable load; a negativestiffness element; and a hydraulic system coupled to the positive andnegative stiffness elements and configured to adjust a relative positionof the positive and negative stiffness elements in response to a changein the variable load, while the variable stiffness structure supportsthe variable load.
 2. The variable stiffness structure according toclaim 1, wherein the positive stiffness element is configured to couplethe variable load to an external body, and wherein the negativestiffness element is configured to isolate vibrations of the variableload from the external body.
 3. The variable stiffness structureaccording to claim 2, wherein the change in the variable load produces adisplacement in the relative position of the variable load and theexternal body exceeding an operational range of displacement withinwhich the negative stiffness element provides a negative stiffnessconstant to an aggregate stiffness constant of the variable stiffnessstructure, and wherein, in response to the change in the variable load,the hydraulic system is configured to return the variable stiffnessstructure to the operational range of displacement by applying fluidicpressure to adjust the relative position of the positive and negativestiffness elements.
 4. The variable stiffness structure according toclaim 1, wherein the variable stiffness structure is configured tomaintain a substantially constant stiffness as the relative position ofthe positive and negative stiffness elements is adjusted in response toa change in the variable load.
 5. The variable stiffness structureaccording to claim 1, wherein the positive stiffness element isconfigured to provide a positive stiffness constant to an aggregatestiffness constant of the variable stiffness structure.
 6. The variablestiffness structure according to claim 1, wherein the hydraulic systemcomprises an actuator and is configured to disengage the negativestiffness element prior to adjusting the relative position of thepositive and negative stiffness elements, and to re-engage the negativestiffness element after the adjusting the relative position of thepositive and negative stiffness elements.
 7. The variable stiffnessstructure according to claim 1, wherein the hydraulic system comprises:a hydraulic chamber coupled to the negative stiffness element; anaccumulator; and a valve system configured to move fluid between thehydraulic chamber and the accumulator, wherein the hydraulic chamber isconfigured to exert fluidic pressure on the negative stiffness elementto adjust the position of the negative stiffness element relative to thepositive stiffness element, in response to the change in the variableload.
 8. The variable stiffness structure according to claim 7, whereinthe valve system is configured to increase or decrease the fluidicpressure inside the hydraulic chamber according to the change in thevariable load.
 9. The variable stiffness structure according to claim 7,wherein the valve system comprises two or more pressure relief valvesarranged in opposite directions and configured to permit flow of fluidbetween the hydraulic chamber and the accumulator, when a fluid pressurein either the hydraulic chamber or the accumulator exceeds a reliefpressure.
 10. The variable stiffness structure according to claim 9,wherein the two or more pressure relief valves comprise passive valves.11. The variable stiffness structure according to claim 7, wherein thevalve system comprises one or more of a passive valve and an activevalve.
 12. The variable stiffness structure according to claim 1,wherein the hydraulic system comprises: a first hydraulic chambercoupled between the negative stiffness element and the variable load; asecond hydraulic chamber coupled between the negative stiffness elementand a body; and a valve system configured to move fluid between thefirst hydraulic chamber and the second hydraulic chamber to adjust theposition of the negative stiffness element relative to the positivestiffness element, in response to the change in the variable load. 13.The variable stiffness structure according to claim 1, wherein thepositive stiffness element comprises a first rubber disc and a secondrubber disc, and is coupled to the variable load through an inner post.14. The variable stiffness structure according to claim 13, wherein thenegative stiffness element comprises a pair of buckled discs having afirst end coupled to a body and a second end configured to slide along alength of the inner post.
 15. The variable stiffness structure accordingto claim 14, wherein the hydraulic system comprises: a first hydraulicchamber between the first rubber disc and the negative stiffnesselement; a second hydraulic chamber between the first rubber disc andthe negative stiffness element; and a valve system configured to movefluid between the first hydraulic chamber and the second hydraulicchamber to adjust the position of the negative stiffness elementrelative to the inner post, in response to the change in the variableload.
 16. The variable stiffness structure according to claim 15,wherein a stiffness of the negative stiffness element remainssubstantially constant as the valve system adjusts the position of thenegative stiffness element relative to the inner post.
 17. The variablestiffness structure according to claim 1, wherein the hydraulic systemcomprises: a first hydraulic chamber coupled between the negativestiffness element and the variable load; a second hydraulic chambercoupled between the negative stiffness element and a body; a firstaccumulator; and a valve system configured to move fluid between thefirst accumulator and the first and second hydraulic chambers, whereinthe each of the first and second hydraulic chambers is configured toexert fluidic pressure on the negative stiffness element to adjust theposition of the negative stiffness element relative to the positivestiffness element, in response to the change in the variable load. 18.The variable stiffness structure according to claim 17, wherein thevalve system is configured to increase or decrease fluidic pressuresinside the first and second hydraulic chambers according to the changein the variable load.
 19. The variable stiffness structure according toclaim 17, wherein the valve system is configured to balance fluidicpressures between the first and second hydraulic chambers according tothe change in the variable load.
 20. The variable stiffness structureaccording to claim 17, further comprising a sensor coupled to one of thefirst and second hydraulic chambers, wherein the sensor is configured tosense a fluid pressure inside one of the first and second hydraulicchambers, and wherein the valve system is further configured to controlthe stiffness of the negative stiffness element according to the sensedfluid pressure.
 21. The variable stiffness structure according to claim17, wherein the hydraulic system further comprises a first actuatorconfigured to push fluid from one of the first and second hydraulicchambers into another one of the first and second hydraulic chambers.22. The variable stiffness structure according to claim 17, wherein thehydraulic system further comprises a second actuator configured toincrease or decrease a fluidic pressure in the first accumulator. 23.The variable stiffness structure according to claim 17, wherein thehydraulic system further comprises: a second accumulator, wherein thevalve system is further configured to move fluid between the secondaccumulator and the first and second hydraulic chambers.
 24. Anisolation system comprising: a body; a variable load; and the variablestiffness structure of claim 1 coupled to the body and the variable loadand configured to isolate vibrations of the variable load from the bodyin presence of a change in the variable load.
 25. The isolation systemaccording to claim 24, wherein the hydraulic system comprises: ahydraulic chamber coupled to the negative stiffness element; anaccumulator; and a valve system configured to move fluid between thehydraulic chamber and the accumulator, wherein the hydraulic chamber isconfigured to exert fluidic pressure on the negative stiffness elementto adjust the position of the negative stiffness element relative to thepositive stiffness element, in response to the change in the variableload.
 26. The isolation system according to claim 24, wherein thehydraulic system comprises: a first hydraulic chamber coupled betweenthe negative stiffness element and the variable load; a second hydraulicchamber coupled between the negative stiffness element and the body; avalve system configured to move fluid between the first hydraulicchamber and the second hydraulic chamber to adjust the position of thenegative stiffness element relative to the positive stiffness element,in response to the change in the variable load.